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It's only happened to me once so far, but in my case the smell was the varnish/coating breaking down, and it wasn't long before the windings shorted out. I don't know of anything that could be done, other than re-winding it. I'm keeping an eye on this thread, myself, because I'd like to know whether there's anything else that can be done as well.

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But don't run a series motor.unloaded, they overspeed and dismantle themselves...

As for why, unloaded there's very low current in the armature (as it's only necessary to overcome friction), so little current in the field coil, so less back-emf to limit the armature current, so the speed rises until the back-emf starts braking the armature, usually about 3 seconds before centrifugal forces tear the windings loose...

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Usually by the time they are smelling, it's too late to salvage. I've rewound a lot of motors, but where they run upwards of 5000 rpm or so, balance is critical. I always epoxy my windings, often including a wrap of tensile cord, but the final balance is a dynamic thing which varies from end to end of the armature. It's a real PIA to do without a proper machine.

If you're lucky and the armature is good (seldom is that the case) then you can rewind the field coils- all it takes is time and care.

For something like a vacuum cleaner motor, you'd just replace it if it's gone. If it isn't gone, you're not likely to be robbing it -

I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my pass​ion-

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At the first hint of varnish smell or even change in sound, check for plugging somewhere in the suction/blower circuit since most vacuum cleaner motors are cooled by the suction air flow. On a friends vacuum I discovered that the blower impeller/wheel was completely filled with lint. I don't know how that happened unless they ran it without the filter/bag. Anyway, be sure everything is clear and try to run it. In the "good old days" before better varnishes were common, I sometimes soaked questionable motors in orange shellac and baked them. Some ran for years.

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I would suggest to not epoxy the armature windings, unless they are actually loose- then use a very minimal amount and try to put about the same amount on both sides- balance being the main issue. If the insulation is ready to flake off, then you have nothing to lose by epoxying it.

You can clean the armature though- brake clean works pretty good for that. You can even do the soapy water thing. I'd have no problem with that, and I'd rinse it in hot running water as well. The trick is to get the job done, then bake it at low heat to completely flash off any remaining moisture. You'll have to remove the bearings, or totally seal them away from the water while you wash it. If the commutator is worn, you'll have to turn it. This is best done with the commutator end supported in a steady rest, because any runout here is bad news. If the comm is offset the brushes will bounce and things will not go well from there.

If the bearings are getting on the noisy or rough side, they won't last long from this point.

Hopefully the smell is coming from a contaminant which can be washed away. If it's from the insulation- well, I'd say the chances of mitigating that are slim.

If you have never taken a motor apart, this will be an education. Sometimes there isn't a way to get it apart, often you won't be able to remove one of the bearings from the housing, or pull the armature shaft free of the bearings. The brushes, being spring loaded, will shoot out of their bores and will be a pain to get back in and the armature back in without damaging them. If they can be removed from the outside, do that before taking the motor apart. Label them so they will go back in the same as they were- assuming they and the commutator are ok as is. If you're going to resurface the commutator and replace brushes, it's best to seat them to the comm before running the motor under power. You usually wrap a strip of sandpaper around the comm, grit side out, then pull the paper back and forth while the brush bears down on it under spring pressure. Do this for both brushes, then clean the whole area really well afterwards. If you don't properly prepare things, motor life will be shorter than you'd like.